NTT has developed a prototype fuel cell small enough to fit inside mobile phones and other portable consumer electronics devices. It plans to be able to sell it commercially within three years.

The prototype is a micro polymer-electrolyte fuel cell (PEFC) that works by combining hydrogen with oxygen, generating electricity and water, and is more powerful than the direct methanol fuel cells (DMFCs) currently being developed by many companies, said Kazuya Akiyama, a researcher at the energy systems project at NTT's energy and environment systems laboratories.

The power density of the NTT cell - a measure of the amount of power it can generate relative to its size - is up to 200 milliwatts per square centimetre. That would provide a 3G phone that uses 2.5 watts of power with about 9 hours of talk time, Akiyama said.

In contrast, a DMFC developed by NEC last year offered a power density of 70 milliwatts per square centimetre. NTT calculates that to match the size of lithium ion batteries used in mobile phones, a fuel cell must have a power density of about 160 milliwatts per square centimeter or more, he said. "DMFCs can't do it. There isn't enough power," Akiyama said.

NTT believes that the extra power advantage means hydrogen-fuelled PEFCs will be able to replace lithium ion batteries inside mobile phones and in tests the prototype has been able to power a phone long enough to allow a video or voice call, he said.

NTT's prototype is currently 13mm by 42mm by 80mm, and weighs 104 grams. It will take two years for the company to shrink it to so fit inside a mobile, Akiyama said.

The hydrogen-fuelled PEFC technology does have at least one disadvantage compared to DMFC technology, he said. Methanol fuel cells can work off small, non-pressurized cartridges of the liquid while hydrogen-fuelled PEFCs require pressurized hydrogen gas. While the hydrogen is only pressurized to 2 or 3 atmospheres, the industry has yet to create a small, safe and standardised container for this.

NTT has designed a hydrogen storage unit that is slightly bigger than an automobile battery that can store 50 litres of hydrogen. In the home, it could be used as a refuelling station for a number of types of fuel cells although is too big for portable use.

Building a supply and container infrastructure, creating international packaging standards and making a legal framework to allow transportation of hydrogen canisters will take about three years, a year longer than it will take to resolve similar issues with methanol, Akiyama estimated. Regulations to allow passengers to carry methanol fuel canisters onboard commercial airliners should be completed around 2007.

"The fuel canisters we are using are very heavy and we need to make them smaller and lighter, and safety is a big concern," he admitted.